Why do we need to equalize lead acid batteries? Read on to find out how important regularly doing so is.
About eight years ago, we switched to Absorbed Glass Mat (AGM) batteries on Morgan’s Cloud, to get the following benefits over liquid filled lead-acid batteries:
- Shorter recharge times since AGM batteries accept a faster charge rate.
- About 10% more capacity in the same size battery.
However, our experience was not good: We went through four sets of house batteries from two different manufacturers.
In the process of solving that problem, we learnt a huge amount that can be applied to the care of any lead acid battery. Read on:
A known way to charge the battery close to 100% at each cycle without much trouble: split the battery into two separate banks. Each bank is used only every two days and the other bank is charged close to 100% during its “rest day” with solar, wind, engine or generator without increasing much the overall generator working time.
Very good point. The only trouble is that to do that, and still not cycle our batteries deeper than 50%, we would need to double the size of the battery bank, something we don’t have the space to do. Also, if we did double the size of the bank, we might be better served by using the whole bank and only discharging to 25%.
Finally, if we only charged half the bank at a time, we would not be able to fully load the generator since only half the bank would be in high absorbing mode.
Having said that, the system you propose might work great for a boat with lower power requirements than ours and with lots of solar and wind power. It would require some fairly complex wiring to route the charging current to the resting battery while making sure that it did not see any loads.
I think Robert’s advice on splitting battery banks, and maybe not having a starter battery, does not match with current thinking in that it is much more efficient to have just one large service bank and a much smaller starter battery.
There are 4 very good reasons why bigger is better:
1. Doubling the service bank size means the life cycle is longer as the DoD is unlikely to fall so close to 50% so often. Life cycle at 50% DoD may be 1000 charge and discharge cycles. At a DoD of only 25% the life cycle may be 2500 or more.
2. Doubling the service bank size also means the “apparent capacity” is greater. Peukert’s law says that the apparent Ah size of a bank changes depending on the current draw.
A bank is designed to deliver a capacity with a current discharge that will flatten the battery in 20 hours. (The 20 hour rate) So with a 100Ah battery, a 5A load will flatten the battery to 10.5v in 20 hours.
When drawing currents higher than 5 amps the “actual” bank size will be much smaller, so the bank will not last as long before it needs re-charging. Conversely when using much less than 5 amps the bank size will be larger and will deliver more Ah.
If a 100 Ah that battery has a Peukert value of 1.25, then higher or lower loads than 5 amps will change the actual capacity of the battery by the following amounts.
With a 10A load for 20 hours there are only 84Ah’s in the 100 Ah bank.
With a 1A load for 20 hours there are 150Ah’s in the 100 Ah bank.
3. Doubling the service bank size also means it will be more efficient and accept more Ah more quickly from all charging sources during the boost phase up to 80%.
It takes a bit of very over-simplified maths to prove the point, but a 100 Ah battery that is discharged to 50% may accept 20Ah in the first hour during the boosts stage, maybe 10Ah in the second hour during the start of the less efficient absorption phase, and the remaining 20Ah in another 5 hour. Doubling the battery size to 200Ah, with the same charging source of 20 amps, will accept 10Ah into each battery in 1 hour, that’s 20Ah into the bank. In the second hours it will store another 20Ah. That’s 40Ah replaced in two hours, as compared to 30Ah with a single bank. In the 3rd hour it may still accept 20 amps into the bank because a single battery in the start of the absorption phase could accept 10 amps. That’s 60Ah in three hours.The key point is that for two hours it is still in the more efficient boost stage where the battery is taking all the current the charge source can give it. Note that the initial boost charging stage has captured 40Ah in two hours, and 60 Ah in three hours. With the smaller bank it could only capture 20Ah in the first hour during boost and 30Ah after the second hour during the start of absorption. The third hour may add another 5 amps. That’s 35Ah with one batteries and 60Ah with two batteries. So a bigger bank will be more efficient and accept more Ah more quickly from all charging sources.
Since a lot of the time we are only charging up to the absorption stage which is about 80-85% then this increase in stored Ah is significant.
4. If you have a larger bank – or many smaller batteries in one large bank, it is easy when they start failing to just disconnect the bad ones and run on the others as long as you can until you can replace the whole bank. This may also allow skippers to search around and find the batteries they really want – not just be forced to buy the local “rubbish” because they are desperate.
Hi Matt,
All good points that I would agree with. However, there are also advantages to splitting into two banks too:
So, like so many things in voyaging the answer to which is best, one bank or two—I’m talking about splitting the house bank here, since I believe the engine start should always be separate from house—is that oh so unsatisfying “it depends”.
For me, I like to have the house bank split because it gives me the best of both worlds since most of the time I leave the switch that splits them on “both”, to get the advantages you explain so well, but when it makes sense I can split them up, say when equalizing.
A switch to split when you need to is a very good solution for equalizing. I just EQ one at a time by disconnecting the others. I think the advantages from the Peukert effect of having effectively a much larger bank means the batteries will last much longer. I have only 140 watts of solar that supplies the daily needs, and 400W wind. We have to find some shorepower every 3 weeks to get the bats to 100% – in between the 280 A DC genny and the motoring which we seem to do a lot of here in the med. My 1050 Ah Lifelines are now in their 10th year and standing up very well to individual 10 hour load tests right now. I only EQ them at the beginning and end of the season.
I’m as surprised as you were, John, that your AGMs would be dying so quickly. Hopefully your experiments will reveal the cause of the problem- I’m betting on charger programming, but that’s far from the only possibility.
More sophisticated batteries inevitably bring with them more potential problems and the need for more advanced monitoring systems. Taken to the extreme, this results in the lithium polymer battery packs we used to use on solar cars, and that have now evolved for modern electric cars: 5 kWh or so from a 30 kg battery (the equivalent of over 400 amp-hours at 12 V), but each string in the pack needed real-time monitoring for over-voltage, under-voltage, over-current and over-temperature- any of which could result in either a string failure or a cell breach.
One thing that those Li-Poly experiences taught me was that, whenever possible, it’s best to obtain protection and charging circuitry from the same engineering team that designed the battery. If that’s not possible, then the different suppliers have to be willing to work together and provide each other with detailed specifications and all the characterization curves for their respective products. The battery guy, for example, should be able to give the charger guy a book of graphs relating voltage, state of charge, input or output current, charge acceptance rate, etc. for all foreseeable operating conditions.
You are absolutely right. As you will see as the series unfolds, the big problem is that the battery and charger manufacturers, in the marine business, are not on the same page.
I have a friend living on his sailboat without shore power for several years. Only Solar panels and diesel generator. He uses standard industrial positive tubular motive power batteries, with two banks as previously mentioned, one day rest and 100% charged, one day in use and partially charged. After 7 years and about 1300 cycles, these batteries contain still about 70% of the initial Ah value.
The life time of this type of industrial batteries (2 volts elements) is about 1500 cycles at 50% discharge and C/5 discharge current, or 10 years floating. Their cost is much lower than AGM or gel batteries. About 1.5 – 2€ per Ah at 12 volt.
Thanks for the really good real world data. I’m sure it will be really useful to our readers.
Unless I’m missing something, this system requires a battery bank total capacity of four times daily use? That is unless the user is willing to run the generator more than once a day. For boats that can fit that number of batteries in, it would seem one of the best systems.
There is one other point: Most generators run at constant RPM and near constant fuel burn, regardless of load, therefore such a system will only be fuel efficient if the generator is sized to be fully loaded when charging half the bank (plus other loads) in absorb mode. Since the smallest diesel generators are around 5Kw this would, once again, imply about a 1000 amp hour battery bank at minimum. The other option would be a small gas generator, like the Hondas, or one of the newer technology variable RPM generators.
All of the above shows the importance of a total systems approach in all of this.
Hello All,
I am Justin Godber with Lifeline Batteries. I have been working with John and following this blog. I thought I would start by responding to some of the topics above and clarifying a few things and answer some of the questions that will follow.
AGM batteries ARE a lead acid battery. So are GEL batteries. They all just contain the electrolyte in different ways. There are three types of lead acid batteries: Wet Cell batteries, GEL Cell batteries and AGM batteries.
Wet cell batteries as we all know are the type that you have to refill with water. They are messy and can be more dangerous because of the volume of hydrogen that is emitted during recharge.
GEL batteries have taken wet electrolyte mixed with silica sand to make a GEL. We used to make these until about 1989. As most people think this is a “newer technology”, really it is quite old and as I stated we actually stopped making these in 1989. GEL batteries are sealed and work well with very strict charging regimes. The biggest problem with GEL batteries is the charging and the vibration. With vibration the GEL forms all these small air bubbles. Similar to what you would see in a bottle of hair gel. These air bubbles virtually cannot go anywhere so they stay in the GEL. All is fine until all these bubbles sit against the battery plate. Any and all bubbles that are against the plate will not be able to produce any capacity because there is air there, not electrolyte. This may not sound like a big deal but there could be thousands of bubbles in there covering more than 50% of the plates. Secondly, the charging. Charging GEL batteries can be very temperamental. GEL batteries require very strict charging voltages and cannot really deviate 1/10 of a volt either way to avoid premature death.
AGM Batteries. This is important. NOT ALL AGM BATTERIES ARE CREATED EQUAL. AGM batteries have all the electrolyte absorbed into a fiberglass matting. They are then charged and formed and then all the excess acid is dumped out. We then seal the caps on the battery permanently. This results in a completely sealed battery. You can charge these batteries with 100% of their amp hour rating. This is a big advantage. You can charge a 100 amp battery with 100 amps. In fact they actually respond better in lab conditions when they are charged up faster. A Wet cell and GEL cell can only take 35% of their rated capacity on recharge. Making an AGM battery is like making a cake. The recipe has to be just right. We take pride in our batteries, we make everything (proudly) in the USA, and I mean everything. We also manufacture everything by hand. We have 17 quality checks as we are going down the line. We make batteries for Marine, RV, Aircraft, and Solar industries. We make a true deep cycle battery for the marine industry. Besides being very expensive to manufacture we really have no cons over any of the aforementioned battery types.
Now that all battery types have been explained, here is the part you have been waiting for. ALL batteries need to be fully recharged to avoid sulfation build up on the plates. I am not sure if I can post links on here so before I do I am asking. I can send links for Trojan Battery, Deka Battery, Odyssey Batteries, etc…They all state the same thing. Batteries must be fully recharged to avoid damage and premature failure. This is why:
As I mentioned before these are all lead acid batteries. They all perform the same chemically when charging and discharging. These batteries are all made from lead and lead dioxide and electrolyte. When the battery is discharged the plates go under a chemical reaction called lead sulfate. When the batteries are recharged this reaction is reversed. This reversal changes the plates from lead sulfate back to lead and lead dioxide. When the batteries are left to sit in a discharged state the lead sulfate does not get reversed and starts to harden, or crystallize. When you look at it under a microscope it looks like crystals. The longer it sits like that the harder it gets and slowly starts to grow farther around the plates. This is the part where I will tell you how sailors eventually ruin batteries.
Trust me, if I was in most of your positions I would probably do the same thing even knowing what I know. Batteries are not like a fuel tank. You cannot refill them to 85% and expect to always have 85%. As I stated the hardened sulfate will start growing. So when you use the 50-85 rule it works great for the first six months and then as the resistance starts to build and the sulfate starts to grow it goes 50-84 and then 50-83 and then 50-82 etc…Even though your charger says you are back to 85% it doesn’t really know because the resistance starts confusing the charger. It thinks it is back to 85% when it is slowly deteriorating. Eventually you will not be able to get the batteries above 12.2 volts and then we get a phone call.
There are a few solutions to avoiding this scenario. The easiest one for us, but not for you, is fully recharging every time. This will keep the batteries healthy all their life.
The other scenario when cruising is to use the 50-85 rule but you must equalize your battery bank once or twice a month. This will stop the sulfate from hardening as much as it would normally. John is currently using a similar scenario as field and we have had success in the past with some Trans-Atlantic crossings and they end up on the other side of the pond with fully charged batteries.
That last paragraph will bring up the next question. “I thought you couldn’t equalize AGM batteries”. Well, as I stated earlier ALL AGM BATTERIES ARE NOT CREATED EQUAL. I can only speak for our batteries but you can equalize them. It is a great tool to use on the aforementioned scenario. Also a great tool just in general to help clean off the plates and gain some capacity back.
Sailors have always struggled with all this battery/battery charging and we know why. We also know why you will only charge to 85%. As I stated I probably would do the same thing but we have been working and simulating your scenarios in the lab for years and we think the program that John is on is going to be successful.
I want to write so much more but I will wait for questions, concerns and comments so I can be more specific.
-Justin Godber
Lifeline Batteries
Could someone give me a two-sentence explanation of equalization for an AGM battery? (what it is and how it is done?)